Examples of the secondary battery include a nickel-metal hydride battery, a lead-acid battery, and a lithium-ion secondary battery.
Such secondary batteries are used as power sources in portable information terminals typified by mobile phones. In particular, lithium-ion secondary batteries have been actively developed because capacity thereof can be increased and size thereof can be reduced.
In a lithium-ion secondary battery, as a positive electrode active material, a phosphate compound having an olivine structure and containing lithium and iron, manganese, cobalt, or nickel, such as lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), and lithium nickel phosphate (LiNiPO4), which are disclosed in Patent Document 1, has been known.
Layered rock-salt compounds such as LiCoO2 and Li2MnO3 and spinel compounds such as LiMn2O4 are known as positive electrode active materials. Not only the behavior of a battery when those compounds are used as positive electrode active materials but also physical properties such as magnetic properties, have been widely researched as disclosed in Non-Patent Documents 1 and 2, for example.